CAB and ECAB members

Answer:
Generally, you can do that, but practically - it's not approach I'd advise. Namely, ECAB needs rush action, quick decision and does not have time to wait for (usually - regular) meeting, long discussions (which usually happens when you have too big group of people), brainstorming, testing result analysis...etc.
Most of the activities are, in some form, typical for CAB (Change Advisory Board - i.e. authorizing body for normal changes). ECAB needs different approach so therefore I think you need different approach.

These articles can help you:
"How to manage Emergency Changes as part of ITIL Change Management" https://advisera.com/20000academy/blog/2016/01/19/how-to-manage-emergency-changes-as-part-of-itil-change-management/
"Change Advisory Board in ITIL – advise, approve or what?" https://advisera.com/20000academy/knowledgebase/change-advisory-board-itil-advise-approve/